This invention relates generally to memory devices and, more particularly, to solid-state memory devices having reduced sensitivity to single event radiation effects.
Solid-state memory devices are widely used in computers and other electronic devices onboard spacecraft and must be able to operate reliably in the harsh radiation environment of space. Memory devices are generally subjected to two types of radiation effects in space, one caused by overall or total dose radiation and the other by high energy particles. Total dose radiation typically causes slow, overall damage to memory devices, such as aging, and high energy particles cause single event upset and latchup. Single event upset is usually a nondestructive type of event in which a stray charge is deposited in the device, causing one or more of the memory locations to be disrupted. Single event latchup is a potentially destructive type of event in which abnormally high amounts of supply current are drawn by the device, causing loss of data and possible catastrophic damage. A solid-state memory device can be shielded from total dose radiation by a lightweight metal, such as aluminum, but not from high energy particles.
Static random access memory (SRAM) devices have typically been employed as memory storage devices in the space environment because of their reduced sensitivity to radiation effects. However, SRAM's are expensive and, at the present time, cannot be fabricated in dense configurations. Dynamic random access memory (DRAM) devices are much less expensive than SRAM's and can be fabricated in dense configurations. However, DRAM's are much more susceptible to radiation effects than SRAM'S, and have typically employed redundancy to prevent single event upset and external circuitry to detect single event latchup. The external circuitry cuts power to any memory device in which a latchup is detected. The memory device can then be repowered, but the data is lost. Accordingly, there has been a need for a solid-state memory device that is not susceptible to single event radiation effects. The present invention is directed to this end.